Other Space Geodetic Techniques E. Calais Purdue University - EAS Department Civil 3273 ecalais@purdue.edu
Satellite Laser Ranging Measurement of distance (=range) between a ground station and a satellite = Satellite Laser Ranging (SLR) Ground station transmits a very short laser pulse from a telescope to a satellite The laser pulse is retro-reflected by corner cube reflectors on the satellite back to the ground telescope Very precise clock at the ground station measures the round trip time t emission t reception Time measurement accuracy < 50 picoseconds, or < 1 centimeter in range 3 stations, 1 satellite => position of the satellite (if station position known) 3 satellites, 1 station => position of the station (if satellite orbit known) Tracking a satellite with a network of SLR stations SLR at the Goddard Geophysical and Astronomical Observatory. The two laser beams are coming from the network standard SLR station, MOBLAS-7 (MOBile LASer) and the smaller TLRS-3 (Transportable Laser Ranging System) during a collocation exercise.
Satellite Laser Ranging Geodetic satellites commonly used in SLR: Starlette (France, 1975) Lageos-1 (US, 1976) Etalon-1,2 (USSR, 1989) Topex/Poseidon (US/France, 1992) Lageos-2 (US/Italy, 1992) Stella (France, 1993) GPS-35,36 (US, 1993/94) Glonass-63,67 (Russia, 1994) ERS-2 (ESA, 1995) GFZ-1 (1996) MIDORI/ADEOS (Japan, 1996) TiPS (US, 1996) ERS corner cube array SLR station at Calern, France Starlette, a geodetic satellite Launched in 1975 48 cm diameter, 47 kg
Lunar Laser Ranging = SLR to the moon (first achieved in 1969) LLR station distribution Location of laser reflectors in the Moon Lunar corner cube array (Apollo XIV) Lunar laser station at Calern, France
Pros: Absolute and direct measurement of satellitereceiver distance Cons: Expensive Heavy operation Difficult to automate global coverage poor Applications: Orbit determination Earth s gravity field Ocean altimetry Precise positioning of ground stations Geophysics Geodesy Satellite Laser Ranging
Very Long Baseline Interferometry = VLBI Radio-astronomy technique, used to locate and map stars, quasars, etc = sources Measures the time difference between the arrival at two Earth-based antennas of a radio wavefront emitted by a distant quasar Signal = noise, wavelength = 1-20 cm If the source positions are known ground baseline geodetic VLBI Time measurements precise to a few picoseconds, relative positions of the antennas to a few millimeters
VLBI Cryogenic receiver VLBI antenna at Algonquin, Canada Hydrogen maser Mark III correlator
VLBI The astronomic sources of geodetic VLBI (e.g. quasars) are located billions of light years away from Earth: They appear point-like, with no motion No need for modeling their motions (cf. satellite orbits) less errors Only technique capable of establishing a direct link between the quasi-inertial frame (radio sources) and the terrestrial reference frame Only technique capable of measuring all components of the Earth's Rotation directly: Variations of the Earth's spin axis in space (precession, nutation) Variations of the Earth's spin axis relative to the Earth's crust (polar motion) Rotational velocity and phase (Universal Time, UT).
VLBI VLBI site distribution Pros: The most precise and accurate space geodetic technique Direct link between inertial and terrestrial frames Cons: Expensive Heavy operation Difficult to automate global coverage poor Applications: Reference frames Geophysics Provides precession, nutation, polar motion, UT1
VLBI
Doppler DORIS station in Badary, Siberia Orbitography DORIS (France), PRARE (Germany): Doppler orbitography, Receiver in the satellite, emitter on the ground Satellite records data and downloads it to a data center (centralized system) DORIS on Spot 2, 3, 4, on ERS1 and 2, on Topex- Poseidon, on EnVISAT, on Jason Excellent geographic coverage DORIS network
GPS GLONAS S Orbital planes 6 6 GLONASS Orbit inclination 55 64.8 Orbit height 20200 km 19100 km Carrier frequency L 1 : 1575.42 MHz L 2 : 1227.60 MHz L 1 : 1602 + k 0.5625 MHz L 2 : 1246 + k 0.4375 MHz k=1,...,24 Russian GPS First satellite launched in 1982 Codes System time Repeat time CA-Code for L 1 P-Code for L 1 and L 2 GPS-Time Sidereal day CA-Code for L 1 P-Code for L 1 and L 2 UTC(SU) 8 days As of March 2005 = 12 satellites operational Several manufacturers sell GPS/GLONASS receivers www.glonass-center.ru A GLONASS satellite
GALILEO European GPS, + China, + Israel Commercially-oriented system, (GPS was originally military) ~30 launches 2006-2008, operational 2008: 27 operational + 3 spares 3 circular orbits at 23,616 km, inclination 56 degrees L-band, dual-frequency Key difference with GPS: integrity monitoring Commercial services http://europa.eu.int/comm/dgs/energy_transp ort/galileo/index_en.htm GNSS
VLBI-GPS comparison
GPS-DORIS- SLR comparison